FIG. 1 shows a pair of transmitting and receiving semiconductor chips (or units) 101, 102 coupled together by a serial link 110 having a data signal line 113 and a clock signal line 112. The transmitting unit 101 sends a data signal 105 to the receiving unit 102 along data signal line 113. The receiving unit 102 uses a clock signal 106 that is sent along clock signal line 112 to receive the data 105.
That is, in the example of FIG. 1, the receiving unit 102 clocks the data signal 105 on the rising edge of the clock signal 106. The clock signal 106 may be referred to as a quadrature clock because the phase of its rising edges are 90 degrees away from the rising edges of the data signal 105 (using the data signal 105 as a phase reference). A link that transmits a clock along with data may be referred to as a source synchronous interface. Various source synchronous interfaces exist such as, for example, Low Voltage Differential Signalling (LVDS) and Serial Gigabit Media Independent Interface (SGMII).
A problem with serial links, particularly as their frequency of operation rises, is the presence of skew 109 between a data signal 107 and a clock signal 108 when it is received at the receiving unit. Skew 109 is any phase relationship between the edges of the data signal 107 and clock signal 108 other than the nominal or “designed for” phase relationship (such as 90 degrees, using the data signal 105 as a phase reference).
Skew may arise because the transfer function and/or trace length of the data signal line 113 is different than the transfer function and/or trace length of the clock signal line 112. For example if the data signal line 113 is shorter or has less capacitance than the clock signal line 113, the rising edges of the clock signal 108 can have more than 90 degrees of phase shift with respect to the rising edges of the data signal 107.
For a given difference in transfer function and/or trace length between the data and clock signal lines 113, 112, greater skew is observed between the data signal 107 and clock signal 108 as the frequency of operation of the serial link 110 increases. That is, the differences between the signal lines 113, 112 have an effect on the delay of the signals as they propagate from the transmitting unit 101 to the receiving unit 102. As the frequency of the serial link's operation rises, the delay represents a greater percentage of the data signal's pulse widths.
As skew 109 increases the performance of the serial link degrades. That is, because the receiving unit 102 uses the clock signal to clock the reception of the data carried by the data signal 107, the “misposition” of the clock signal 108 edges causes the receiving unit 102 to consistently clock incorrect data.